In many applications, non-contact machining is used for the mass production of parts which must be machined to close tolerances. There are several factors which effect the repeatability of accuracy of machining by the non-contact process. Non-contact machining commonly includes the use of electric discharge EDM, lasers, or electron beam. In the EDM process, factors which effect the repeatability of accuracy include the process parameters of electrode size, spark energy, spark current, gap length and gap voltage. For example, when the EDM process is used to machine a small hole through a thin plate, the variation in diameter of the wire electrode over a large number of parts may result in many parts being out of tolerance unless special measures are taken. Also, mechanical wear in the fixturing system will result in variation in hole diameters. Even though such variations may be on the order of one ten thousandths inch, they nevertheless adversely effect the accuracy of the finished part. In the case of an orifice plate for fuel injectors, for example, the specified tolerance is 1% but the electrode wire commercially available has a tolerance of two percent. Thus, the desired accuracy can not be achieved uniformly by conventional practice.
Accordingly, there is a need for obtaining improved uniformity and accuracy by the non-contact process. Further, it should be achieved by depending upon stringent control of the various parameters to precisely constant values. Instead, as a practical matter, the improved accuracy must be achieved while tolerating variations with time in certain operating parameters of the EDM process.
U.S. Pat. No. 4,725,705, issued Feb. 16, 1988 with a common assignee, discloses a method and apparatus for electric discharge machining with adaptive feedback for dimensional control. Such method and apparatus, while suitable for their intended purpose, can be applied to other non-contact machining systems in which a parameter can be adapted to control erosion of a single feature. Furthermore, the '705 patent discloses machining a plurality of workpieces, measuring the hole size, and monitoring the trend of change of the measured flow and adjusting the spark energy based on that trend. However, use of the adjustment after a plurality of parts or even a single part, does not consistently maintain the hole size with the low acceptability limit of 1%.